Security Device With a Zero-Order Diffractive Microstructure

ABSTRACT

A security Device comprises a zero order diffractive microstructure ( 5 ) buried within a substrate ( 3 ). One or more further optical structures, such as microlenses ( 1 ), may be formed on a surface ( 2 ) of the substrate ( 3 ). The further optical structures modify the optical characteristics of the zero order diffractive microstructure ( 5 ). 
     Various alternatives or additional optical structures and methods of producing them are described in additional embodiments.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims priorityto, U.S. patent application Ser. No. 11/659,559, filed on Feb. 5, 2007,which claims priority to PCT/1B2005/002494, filed on Aug. 3, 2005, andto United Kingdom Application No. 0417422.3, filed on Aug. 5, 2004. Thedisclosures of the above applications are incorporated herein byreference in their entirety.

BACKGROUND

The invention relates to security devices containing zero-orderdiffractive microstructures. Such devices may be used as securitydevices in the fields of authentication, identification and security.

The production of zero-order diffractive microstructures having specialcolour effects—for example, colour change upon tilting and/orrotation—for use as security devices in a variety of applications like(but not restricted to) banknotes, credit cards, passports, tickets,document security, anti-counterfeiting, brand protection and the like isknown. The devices can be in the form of hot or cold transferablelabels, adhesive tags and the like. They can significantly decrease thepossibility of counterfeiting compared to state of the art securitydevices possessing security printing techniques, optically variabledevices (OVDs) like optically variable inks (OVI) or diffractiveoptically variable image devices (DOVIDs), UV/IR fluorescent dyes,magnetic stripes etc.

It is known to use DOVIDs like holograms for anti-counterfeiting ofbanknotes or credit cards. Further magnetic codes or fluorescent dyesare often used to proof the originality of items. Unfortunately it isalready possible to produce high quality counterfeited versions ofdevices using all those techniques. Additionally, DOVIDs possess only alow level of security, as non-experts generally do not know what theholographic image looks like. Therefore there is a need for novelsecurity devices that are more difficult to counterfeit.

OVIs, as disclosed in the U.S. Pat. No. 4,705,356, provide higher levelof security, as it is easier for non-experts to observe a colour changethan a complex image. Although OVI's are also difficult to manufacture,and therefore seem to be secure, their effect can be closely mimickedwith colour-shifting inks that are used for decorative purposes and arecommercially available from several companies (for examplehttp://www.colorshift.com). This decreases the value of OVIs asanti-counterfeiting tool.

In the U.S. Pat. No. 4,484,797 colour filter with zero-ordermicrostructures are described for use as authenticating devices. Whenevenly illuminated with non-polarized, polychromatic light such devicesshow unique colour effects upon rotation and therefore can be clearlyidentified. The possibilities for varying the colour effect are limited(see M. T. Gale “Zero-Order Grating Micro-structures” in R. L. vanRenesse, Optical Document Security, 2^(nd) Ed., pp. 277) and noindividualization of the devices is claimed.

The WO 03/059643 also describes very similar zero-order diffractivegratings for use in security elements. The elements have the samedrawbacks as the filters in the U.S. Pat. No. 4,484,797.

It is an object of the present invention to mitigate one or more ofthese drawbacks of the state of the art.

In a first aspect the invention provides a security device comprising azero-order diffractive microstructure buried within a substrate and afurther structure comprising one or more optical structures formed on asurface of the substrate to be viewed by a user that modifies theoptical characteristics produced by the zero-order microstructure.

The further structure may comprise one or more optical structures suchas prisms or microlenses formed on a surface of the substrate to beviewed by a user.

In one embodiment a second further structure may be formed on a rearsurface of the substrate. A mirror may be formed on or behind the rearsurface of the substrate or the further structure may be reflective. Formaximum effect, the mirror should be preferable placed in a distance ofless than two times the wavelength from the zero-order diffractiongrating (for the visible spectrum, thus less than two microns away).

In an alternative embodiment the further structure may comprise a colourfilter embedded in the substrate or applied to the surface of thesubstrate viewed by the user.

In a further embodiment the further structure may comprise a secondzero-order diffractive grating with a different colour response as thefirst zero-order diffractive grating. This enhance the intensity andchange of the colour spectrum of the device.

In a further embodiment the further structure may comprise a materialembedded in or applied to the surface of the substrate, which materialmay be locally optically modified by irradiating the surface of thesubstrate.

In a second aspect the invention provides a method of manufacturing asecurity device comprising the steps of:

forming a zero-order diffractive microstructure in a substrate;

and forming a further structure in or on a surface of the substratecapable of modifying the optical characteristics as viewed at that oranother surface of the substrate by a user.

The further structure may be formed on the surface of the substrateviewed by the user and comprises an optical structure.

In a first embodiment the further structure may be formed by ink-jetprinting.

In an alternative embodiment the further structure may be formed byembossing.

In a further embodiment the further structure may be formed by providinga colour filter between the surface of the substrate viewed by the userand the zero-order diffractive microstructure or on the surface of thesubstrate viewed by the user.

In one embodiment the further structure may be formed behind thezero-order diffractive microstructure and may be reflective.

A still further embodiment comprises the further steps of:

embedding an optically modifiable material in the substrate;

and locally optically modifying the material.

The method may further comprise the step of irradiating the opticallymodifiable material to modify an optical property thereof. A laser maybe used to modify an optical property.

The present invention enables the provision of methods for modifyingand/or structuring, especially individualizing, zero-order diffractivemicrostructure security devices, for example by adding serial numbers,one- or two-dimensional barcodes, images and the like. In particular,methods for carrying out such modifications after the production processbut on-site, for example, at any point of the supply chain, may beprovided by this invention.

The invention also enables the provision of new methods for massproduction of such devices at low costs.

The present invention also enables measurement and identification ofsuch zero-order diffractive microstructures even with low-cost handhelddevices as described in WO 2004/034338 or inter alia in U.S. Pat. No.6,473,165.

Zero-order diffractive microstructures, particularly gratings,illuminated by polychromatic light are capable of separating zerodiffraction order output light from higher diffraction order outputlight. Such structures, for example, consist of parallel lines of amaterial with relatively high index of refraction n surrounded with (orat least in one half space adjacent to) a material with lower index ofrefraction. The structure acts as a kind of Waveguide. An advantageousproduction method is to emboss the microstructure in a polymer web by aroll-to-roll process and afterwards coat the web with ZnS in aroll-to-roll evaporation process. The material above and below the highindex microstructure can have a different index of refraction. Allmaterials above the microstructure have to be transparent (which meanstransmission T>50%, preferably T>90%) at least in a part of the visiblespectral range. The spacing between the lines should be in the range of100 nm to 900 nm, typically between 200 nm to 500 nm (sub wavelengthstructure). These microstructures possess characteristic reflection andtransmission spectra depending on the viewing angle and the orientationof the structure with respect to the observer (see M. T. Gale“Zero-Order Grating Microstructures” in R. L. van Renesse, OpticalDocument Security, 2^(nd) Ed., pp. 267-287). Other parametersinfluencing the colour effect are, for example, the period y, thegrating depth t, the fill factor f and the shape of the microstructure.Furthermore, the grating lines can be connected or vertically orhorizontally disconnected. The shape of the lines can be rectangular,sinusoidal or more complex. In reflection, diffractive microstructuresoperate as coloured mirrors, in which the colour of the mirror varieswith the viewing angle. As long as the materials used show no absorptionthe transmission spectra are the complement of those in reflection.

A unique feature of such structures is a colour change upon rotation.Supposing a non normal viewing angle, for example 30°, and grating linesparallel to the plane containing the surface normal and the viewingdirection, one reflection peak can be measured which splitssymmetrically into two peaks upon rotation. A well-known example of sucha rotation effect is a red to green colour change (one peaks moves fromthe red to the green part of the spectrum the second peak moves from thered part to the invisible infrared part).

Combining zero-order diffractive microstructures with macroscopicoptical structures (size>2 m) modifies the colour effect and/or enablesthe addition of information to the security device (especially toindividualise it). As the added information is directly connected withthe security device it is as secure as the security device.

One way to do this is to modify the surface of the security device. Forexample, changing the roughness of the surface alters the colour effectdue to changes in the scattering of light. A rough surface leads to avery weak colour effect. Coating the rough surface with a suitable, atleast partially transparent material can restore the colour effect. Onepossible way to coat the surface on-site is to use ink-jet printing.Other possibilities are screen-printing and the like or locally meltingthe material of the rough surface (for example by laser irradiation).Another possibility is to locally roughen (for example mechanically) aflat surface to destroy the colour effect at certain areas. Again amacroscopic structuring can be obtained.

Inhomogeneous coatings of the surface are of particular importance. Byadding optical structures like lenses or prisms and the like on top ofthe surface (see FIG. 1) the colour effect can be restored and/ormodified. Such modification changes the optical path, for example,alters the incident angle. For example, if such lenses are large enoughthe human eye can see different colours at different positions of eachlens as each position shows the colour effect of a different incidentangle. Tilting such a security device produces a multicolour effect. Ifthe lenses are small enough (so called micro-lenses) the human eye willrecognize an average of the different colours and therefore an unusualeffect. The lenses can possess spherical or aspherical shape and theycan be convex or concave. Depending on the type of the macroscopicoptical structures the colour change upon tilting the security devicecan be enhanced or reduced. An asymmetric shape of the macroscopicoptical structures (for example, rod like lenses) even alters the coloureffect upon rotation.

The formation of the macroscopic optical structures depends on thecoating technique and coating parameter (for example viscosity, webspeed), the coating material and the material of the coated surface. Thewettability of the surface is one of the key factors. The structureformation can be optimised by modifying the surface for example byirradiation, plasma treatment, prior primer coatings etc. Themodification can be laterally structured forcing the following coatingto take the same or a similar lateral shape. For example, sphericallenses can be obtained by coating a primer onto the surface, which isnot wettable by the material used for the macroscopic opticalstructures, while leaving round areas free (Moench et. al., J. Opt. A:Pure Appl. Opt., Vol. 6, 2004, 330-337).

Another possible way of producing macro-scopic optical structures on topof the surface is to use the phase separation or the dewetting effect ofpolymer films. Both are known to produce well-defined structures withcontrollable size by a self-assembly-process (Gutmann et. al., Appl.Phys. A, Vol. 74, 2002, S463-S465 and Müller-Buschbaum, J. Phys.:Condens. Matter, Vol. 15, 2003, R1549-R1582).

Another advantageous combination of zero-order micro-structures withmacroscopic optical structures is to provide the surface and/or aninterface and/or the rear (if it is covered by a mirror or is otherwisereflective) of the security device with an asymmetric structure, forexample by hot or cold embossing. These structures can be in the form ofa saw tooth or an asymmetric sinus and the like (see FIG. 2). Amongother things they can change the incident angle. One effect is that itenables recognition of the rotating effect even at perpendicular viewingdirection.

Yet another advantageous combination of zero-order micro-structures withmacroscopic optical structures is to provide the surface and/or aninterface of the security device with a hologram structure. Thus thetypical rainbow effect of holograms is combined with the characteristiccolour effect of zero-order microstructures. Further the colour effectis modified as the hologram structure changes the incident angle oflight.

A further method for modifying the colour effect of zero-ordermicrostructures and/or adding information to the security device is toadd a material with colour filter function between the surface of thedevice and the microstructures. This can be done by printing onto thesurface or by incorporating into the polymer substrate. Such materialsare, for example, all kinds of chromophores including fluorophors,phosphorescent dyes, nano-particle like Q-Dots or metallicnano-particles and the like. The colour filter modifies the spectra ofthe incident light as well as of the reflected light at themicrostructures (see FIG. 3). Thus unusual colour effects can beobtained. Especially fluorophors coated onto (or placed at a shortdistance from) the high index material enable the production of unusualeffects due to the enhancement of the excitation caused by theevanescent field near the Waveguide or additional interference effects.

An advantageous construction is to use bleachable chromophors. Bylocally controlled high intensity irradiation with the desiredwavelength a macroscopic lateral structure can be written into thesecurity device. FIG. 4 schematically depicts the writing of anindividual barcode into a security device by bleaching a chromophoreincorporated into the polymer by a laser. An alternative is to use alaser intensity, which locally destroys the high index material or thesurrounding polymer. If a mirror is placed at a defined distance to achromophore a locally confined melting of the surrounding polymerfollowed by a bubble formation can by obtained due to the occurrence ofa standing wave (similar to the production of CDs). Such bubbles scatterlight and therefore alter or destroy the colour effect.

Yet another advantageous construction is to use photochrome polymers,which irreversibly alter their index of refraction upon irradiation witha defined wavelength and intensity. Thus at irradiated areas a differentcolour appears from that at non-irradiated areas.

Yet a further method for modifying the colour effect of zero-ordermicrostructures and/or adding information to the security device is touse a wet coating process for example flexo-printing, gravure printing,ink-jet-printing or screen-printing, curtain or dip coating, spraying,sol-gel processes (especially UV or thermal curable sol-gel technique)and the like for depositing the high index of refraction material.Possible (but not limited to) organic materials or lacquer containingthem are highly brominated vinyl polymer, nitrocellulose NC, PC, PEI,PEN, PET, PI, polyphenylen, polypyrrol, PSU, polythiophen, polyurethanePU. Other possible materials are inorganic/organic compound materialslike (but not limited to) ORMOCER™ or mixtures of nano-particle andpolymer like (but not limited to) PbS and gelatine. The latter possessindices of refraction up to 2.5 (Zimmermann et. al. J. Mater. Res., Vol.8, No. 7, 1993, 1742-1748). Lacquers containing Al₂O₃ or TiO₂-particleare also possible. If the nano-particles are porous the wet coated layercan be used as a low index material, too. Such a layer consisting ofporous nano-particle, embedded in a polymer matrix, can possess an indexof refraction of down to 1.1.

On one hand such wet coating processes are less expensive than vacuumcoating processes. On the other hand they possess the ability to depositthe layers laterally structured. Especially roll coating processes caneasily produce structured coatings by just structuring the roll (forexample with Logos, text, pictures and the like). FIG. 5 schematicallydepicts a roll-to-roll production process, which combines the embossingof the foil and the printing of the layers.

As most polymers possess an index of refraction below 1.7 the desiredlayered structure can be inverted. In more detail, one wet coated layeracts as the high index material (for example n1.5) and another wetcoated layer as the low index material (for example n1.1). FIG. 6 showsthe multi-layer setup of such an inverted zero-order microstructure.Especially curtain coating is suitable to coat such multi-layers ofdifferent materials not only in one run but simultaneously. Such aninverted multilayer setup avoids the leak of high index polymers.

FIG. 1 shows schematically the printing of micro-lenses 1 on top of arough surface 2 of a substrate 3 by ink-jet-printing. The inkjetprinting head 4 deposits the micro-lenses at desired points on the roughsurface 2. The material used for the lenses fills the valleys of therough surface 2 and forms lenses 1 due to an optimised surface tension.A zero-order diffractive microstructure 5 is embedded in the substrate3.

FIG. 2 a) depicts saw tooth like macroscopic structures 20 at thesurface viewed by a user of a security device 21 having a zero-orderdiffractive microstructure 22 embedded therein.

FIG. 2 b) depicts a sawtooth like macroscopic structure 23 at the rearsurface of the device 21, which is coated with a mirror (b). Thosestructures change the incident and/or emergent angle of light.

FIG. 3 is a schematic drawing of a security device 30 containingchromophores 31 in the polymer 32 between the microstructures 33 and thetop surface 34. The chromophores absorb a certain part of the incomingas well as the outgoing light. In the drawing refraction of the lightwas neglected.

FIG. 4 depicts schematically the writing of a barcode into a securitydevice of the type shown in FIG. 3 by bleaching chromophoresincorporated into the polymer by a laser 40.

FIG. 5 depicts schematically a roll-to-roll production process, whichcombines the embossing rollers 51 (left) and the printing of the highindex material by means of rollers 52 (right). The counter pressurerollers 53 and 54 enable well defined embossing and printed layerthickness.

FIG. 6 shows schematically an inverted layer setup with zero orderedmicrostructures 61. The substrate 62 with relatively high index ofrefraction is coated with a multilayer structure in which themicro-structured high index coating 61 is embedded between two low indexlayers 63 and 64.

FIG. 7 shows schematically a substrate 70 having two consecutivezero-ordered microstructures 71 and 72 one on top of each other forenhancing the colour response. It may be viewed in reflection astransmission.

1. (canceled)
 2. A security device comprising: a homogenous zero-orderdiffractive microstructure to display a colour change on tilting orrotating the homogenous zero-order diffractive microstructure, thehomogenous zero-order diffractive microstructure being buried within asubstrate and having an index of refraction higher than an index ofrefraction of the substrate; and a further structure comprising asawtooth or an asymmetric sinus or prisms or rod-like lenses formed in aregion of the homogenous zero-order diffractive microstructure, thatmodifies the colour change displayed by the homogenous zero-orderdiffractive microstructure.
 3. A security device according to claim 2,in which the further structure enables recognition by an observer of thecolour change displayed upon rotation of the homogenous zero-orderdiffractive microstructure even at a perpendicular viewing direction. 4.A security device according to claim 2, in which the further structurecomprises an asymmetric structure.
 5. A security device according toclaim 4, in which the asymmetric further structure alters the colourchange displayed on rotating the homogenous zero-order diffractivemicrostructure.
 6. A security device according to claim 2, in which thefurther structure changes an incident angle of light entering thesecurity device and/or an emergent angle of light leaving the securitydevice.
 7. A security device according to claim 2, in which the furtherstructure is formed on a surface of the substrate.
 8. A security deviceaccording to claim 7, in which the further structure is formed on a rearsurface of the substrate and is reflective.
 9. A security deviceaccording to claim 2, in which the further structure is formed at aninterface within the substrate.
 10. A security device according to claim9, in which the further structure is behind the homogenous zero-orderdiffractive microstructure and is reflective.
 11. A security deviceaccording to claim 10, in which the reflective further structure is at adistance from the homogenous zero-order diffractive microstructure ofless than two times a wavelength of visible light.
 12. A security deviceaccording to claim 10, in which the reflective further structure is at adistance of less than 2 micrometres from the homogenous zero-orderdiffractive microstructure.
 13. A security device according to claim 2,in which the further structure comprises macroscopic optical structures.14. A security device according to claim 13, in which the macroscopicoptical structures are of size greater than 2 micrometres.
 15. Asecurity device according to claim 2, in which the further structure isformed by embossing.
 16. A method of manufacturing a security devicecomprising the step of: forming a homogenous zero-order diffractivemicrostructure in a substrate, the homogenous zero-order diffractivemicrostructure having an index of refraction higher than an index ofrefraction of the substrate, to display a colour change on tilting orrotating the homogenous zero-order diffractive microstructure; andforming a further structure comprising a sawtooth or an asymmetric sinusor prisms or rod-like lenses in a region of the homogenous zero-orderdiffractive microstructure, that modifies the colour change displayed bythe homogenous, zero-order diffractive microstructure.
 17. A methodaccording to claim 16, in which the further structure is formed as anasymmetric structure.
 18. A method according to claim 16, in which thefurther structure is formed by embossing.
 19. A method according toclaim 16, comprising the step of forming a reflective layer on saidfurther structure.